I don't currently carry a paper planner, but I also used different colors when I did.

I liked to use colored highlighters and outline blocks of time.

Now I run my personal planner in outlook & sync to iPhone and have colors for various purposes - personal, Mary Kay appts, MK prep time, MK phone time, family, kids school etc.

We also keep a 2x3ft whiteboard dry erase calendar on the kitchen wall -that one is mostly for family activities - each person has a color so I can see at a glance who is doing what. Really helped when I had 2 kids in scouts/soccer/art school.

I'm thinking about picking up an iPad or the rumored iPad mini to carry as an electronic planner among other uses.